The invention relates to the field of thermal turbomachines. It relates to a combustion chamber and to a method for the reduction of thermoacoustic vibrations in a combustion chamber according to the preambles of patent claims 1 and 9 respectively.
In annular combustion chambers of turbomachines, under specific operating states selfexcited pressure vibrations are formed, which are caused by the influence on one another of thermoacoustic pressure pulsations and a local release of heat. Such a pressure vibration is designated as a selfexcited combustion chamber pulsation or as instability and presents a serious problem in modern industrial gas turbines since these use premixing combustion. In premixing combustion, a fuel and air are intermixed as homogeneously as possible before combustion and are ignited only after mixing. Combustion is to be as lean as possible, in order to reduce NOx emissions. However, this lean combustion also necessitates a low flame temperature and makes it difficult to stabilize the flame. This applies above all, but not only, to swirl-stabilized premixing burners. If a plurality of burners are arranged in an annular combustion chamber, azimuthal combustion pulsations may occur, that is to say those taking effect along the combustion chamber circumference. The pressure amplitudes resulting from these combustion pulsations may attain magnitudes which put the mechanical integrity of the machine at risk. This is undesirable, and attempts are made to prevent it by various measures.
A known method for suppressing thermoacoustic vibrations is to install what are known as Helmholtz resonators, as shown in xe2x80x9cTechnische Akustikxe2x80x9d [Technical Acousticsxe2x80x9d], Ivar Veit, Vogel Buchverlag, 1996, page 84. These Helmholtz resonators have the disadvantage, however, that they are designed only for a predetermined frequency, and that, when further pulsations with other frequencies occur, further resonators designed for these frequencies have to be installed.
The object of the invention, therefore, is to provide a combustion chamber and a method for the reduction of thermoacoustic vibrations in a combustion chamber of the type initially mentioned, which eliminates the disadvantages mentioned above.
This object is achieved by a combustion chamber and a method for the reduction of thermoacoustic vibrations in a combustion chamber, having the features of patent claims 1 and 9 respectively.
The combustion chamber according to the invention thus has a number b0 of annularly arranged burners, of which a number k of modulatable burners have means for modulating a fuel mass flow, k being k less than b0, and the modulatable burners being arranged in such a way that between every two modulatable burners are arranged in each case a1, a2, . . . ak nonmodulatable burners, and that the values a1+1, a2+1, . . . , ak+1 are not integral divisors of b0.
It thereby becomes possible to damp a maximum number of azimuthal vibration modes by means of a minimum number of modulatable burners. It is not possible, in the present prior art, to foresee without any doubt the occurrence of an azimuthal combustion vibration during the design phase of the combustion chamber. Each pair of modulatable burners may therefore give rise to at least one undesirable vibration or instability which, however, is damped by the other modulatable burner or burners arranged according to the invention.
In a preferred embodiment of the subject of the invention, a highest value of
LCM(b0, a1+1), LCM(b0, a2+1), . . . LCM(b0, ak+1)
is maximum, LCM designating the lowest common multiple.
In a further preferred embodiment of the subject of the invention, the number k of modulatable burners amounts to at least three.
In a further preferred embodiment of the subject of the invention, a1, a2, . . . ak are different from one another.
In a further preferred embodiment of the subject of the invention, the distances between modulatable burners between which exactly one modulatable burner is arranged are also taken into account: the modulatable burners are therefore arranged in such a way that a highest value of
LCM(b0, a1+1), LCM(b0, a2+1), . . . LCM(b0, ak+1),
LCM(b0, a1+a2+2), LCM(b0, a2+a3+2), . . . LCM(b0,ak+a1+2)
is maximum.
The advantage of these preferred embodiments is that they further increase characteristic frequencies of vibrations or modes which may still occur according to the arrangement of the modulatable burners.
In a further preferred embodiment of the invention, the modulatable burners are designed in such a way that they modulate their fuel mass flow by frequencies which are different from characteristic frequencies of modes occurring by virtue of the arrangement of the modulatable burners. All the modulated burners have in each case the property that their modulation frequencies are different from the natural instability frequency of the combustion chamber which is determined by the geometric and thermophysical conditions of the combustion chamber.
In a method according to the invention for the reduction of thermoacoustic vibrations in an annular combustion chamber with a plurality of annularly arranged burners, of which a plurality of modulatable burners have means for modulating a fuel mass flow, the number of burners being b0, in a number k of the modulatable burners the fuel mass flow is modulated, these modulated burners being arranged in such a way that between every two modulated burners are arranged in each case a1, a2, . . . ak nonmodulated burners, and the distances between the burners a1+1, a2+1, . . . , ak+1 are not integral divisors of b0.
The method according to the invention makes it possible to bring about a damping of combustion pulsations in a combustion chamber which is equipped with a plurality of modulatable burners. In a preferred variant of the invention, this is carried out, using constant modulation frequencies, so that, when the combustion chamber is in operation, there is no need for any measurement of pulsations and for complicated regulation.
Further preferred embodiments may be gathered from the dependent patent claims.